This invention relates, in general, to computer memories, and more particularly, to read-only memories.
Read-only memories (ROMs) are widely used in electronic applications which require information to be permanently stored for reference. As an example, ROMs are used in look-up tables for calculators, as character generators, as hardware arithmetic units for computers, and to permanently store program instructions for digital data processors. Conventional ROM techniques utilize one transistor for each storage location of the ROM. The single transistor can, in some manner, be programmed on or off for two distinct states of operation. It would be desirable to be able to make ROMs in a smaller physical size. If the transistor could be programmed such that it had various states of "on"ness then more than two distinct states could be stored in one transistor. If four, eight, or sixteen different levels of "on"ness (or current capacity) were programmed, then two, three, or four respectively binary bits of information could be stored in one transistor.
Accordingly, it is an object of the present invention to provide a read-only memory wherein at least some of the storage locations have more than two distinct states.
Another object of the invention is to provide a read-only memory employing field effect transistors wherein some of the gate electrode regions of the field effect transistors are larger than the gate electrode regions of other field effect transistors within the memory.
A further object of the present invention is to provide a read-only memory having field effect transistors, wherein the current through the field effect transistors is changed by changing a transistor parameter such as the width, length, or threshold to provide different states of "on"ness thereby providing memory cells having more than two distinct states.